So what is a mistake of law, you ask? For example, if a police officer pulled someone over because he/she thought the person's windshield was cracked, and it was not cracked, that's a mistake of fact. If, however, he/she made the stop because of a cracked windshield, and cracked windshields are not even illegal, that's a mistake of law.
HEIEN v. NORTH CAROLINA is the case, handed down just yesterday. Heien gives the police more leeway to stop cars for the wrong reason, and that SUCKS. In Heien, a police officer stopped a vehicle under the mistaken belief that having only one brake light was illegal in North Carolina. It wasn't illegal. This was a mistake Chief Justice Roberts found "objectively reasonable". Cocaine was later found in the vehicle. The North Carolina Supreme Court reversed a lower appellate court, which ruled that the cocaine had to be suppressed because there was no grounds for the stop. SCOTUS AFFIRMED.
The holding in Heien states that, at least under the US Constitution, both mistakes of fact and mistakes of law are now excusable so long as they are reasonable mistakes, and suppression of evidence will not be required when a such mistake is made. I call this tantamount to a "good-faith exception" for traffic stops.
Prior to this ruling, reasonable mistakes of fact by police were generally excusable, but reasonable mistakes of law were not. In Wisconsin, that has been the law for over 15 years. State vs. Longcore, 226 Wis. 2d 1, 594 N.W.2d 412 (Ct. App. 1999). That principle was recently reiterated in State v. Brown, 2014 WI 269. Therefore, if the police made a stop on a mistake of law and discover evidence as a consequence, that evidence must be suppressed, even if the mistake was reasonable.
Heien does not necessarily mean that reasonable mistakes of law are now excusable in Wisconsin. The Wisconsin court can interpret the Wisconsin constitution in a way that is more generous to citizens' rights than the US Constitution has been interpreted by SCOTUS. However, the Wisconsin court has generally followed the interpretations of SCOTUS on issues of Fourth Amendment law when deciding the scope of Article I, section 11, Wisconsin's counterpart to the Fourth Amendment. Therefore, the decision does not bode good things for Longcore and Brown.